Geothermal brine and steam reservoirs exist in many areas of the world and are a valuable energy resource. Some steam reservoirs yield a superheated steam which, after treatment to remove contaminants, can be used to power a turbine connected to an electrical generator. More usually, the reservoir yields a geothermal brine which must be flashed to produce steam to power the turbine. After powering the turbine, the exhausted steam is condensed in either a direct contact condenser or a surface condenser (e.g., a shell-and-tube-condenser) to produce steam condensate. The steam condensate is then, in the vast majority of cases, used as liquid water make-up to a cooling tower which provides the working fluid (i.e., the cooling medium) for condensing steam in the condenser.
Cooling in the cooling tower is accomplished by evaporation, which produces, as a side effect, the concentration of salts, minerals, and chemicals in the non-evaporated water. If a direct contact condenser is used, as is the case with most geothermal power plants, the cycles of concentration in the cooling tower are normally not controlled. If a surface condenser is used, such as a tube-and-shell condenser, the cooling tower is usually operated to control the concentration of salts by maintaining the cycles of concentration within predetermined limits. This is accomplished by controlling the cooling tower blow down, i.e., controlling the rate at which liquid water is discharged from the cooling tower system.
Depending upon the location of the geothermal power plant, the percentage of total condensate produced in the turbine-condenser system which eventually is evaporated or discharged as cooling tower blowdown is between about 70 and 95%, leaving about 5 to 30% of remaining condensate for disposal, e.g., discharge to the environment, as by distribution upon the earth's surface or discharge to a creek or river. Alternatively, this remaining condensate could be used for a beneficial purpose, e.g., industrial water or agricultural irrigation. Alternatively again, this remaining condensate could be re-injected into the geothermal resource formation, and in many cases this is desired to maintain the resource pressure and volume. However, in other cases, it is not desired, but becomes a necessity because the condensate contains one or more components in excess of applicable environmental discharge regulations. The cost of complying with such regulations--i.e., the cost to construct one or more re-injection wells and the associated surface facilities--is quite high, on the order of $4 million.
One component dissolved in the remaining condensate which may force the operator of a geothermal power plant to bear the cost of re-injection to comply with environmental regulations is boron. Geothermal brines and steam typically contain boron, and as a result the steam condensate obtained in the condenser contains boron.
Ironically, if the boron in the condensate could be controlled to low levels, its presence would actually be beneficial. Boron is one of sixteen important micro-nutrients needed for healthy crop growth--a factor favoring its presence in waters intended for agricultural purposes. On the other hand, boron in forms concentrated above the micronutrient level can inhibit starch formation and in yet higher concentrations prove toxic to plants. The Water Encyclopedia, Second Edition, by van der Leeden et al., Lewis Publishers, Inc. (1990), herein incorporated by reference in its entirety, specifies in Table 6-46 a 0.5 mg/l concentration as the "threshold level" below which the concentration "should be satisfactory for almost all crops and almost any arable soil." The "limiting concentration," "at which the yield of high-value crops might be reduced drastically, or at which an irrigator might be forced to less valuable crops" is identified as 2.0 mg/l. These limits are consistent with the data in Table 6-49 of van der Leeden et al. wherein the "permissible limits" for boron are broken down by crop group. For those most tolerant to boron, e.g., onion, asparagus, and date palm, the permissible limits are between 2 and 3 mg/l. For semi-tolerant crops, such as sunflower, potato, wheat, corn and lima bean, the permissible limits are between 1.33 and 2 mg/l. And for the most sensitive crops, such as pecans, plum, apple, and most especially citrus and avocado, the permissible limits are from 0.67 to 1 mg/l--values which are very much in line with the proposed 0.6 to 1 mg/l limits proposed for Federal drinking water regulations. See "An Update of the Federal Drinking Water Regs," by Pontius, Journal AWWA, pp. 48-57, February, 1995, herein incorporated by reference in its entirety; see especially Table 6 on page 56.
Due to the sensitivity of many crops to the presence of boron, the boron concentration in water used for agricultural purposes often must comply with local water quality regulations. Citrus are among the most sensitive receptors to boron and are adversely affected at a level of 0.75 mg/l--the limit for irrigation water in the Philippines (and others). See in particular pages 4468 and 4473 of the Philippines NPCC Rules and Regulations, Official Gazette, Vol. 74, No. 23, pp. 4467-4476, (1978) herein incorporated by reference in its entirety. In addition, the Philippines, per the Ministry, Bureau and Office Administrative Orders and Regulations, Official Gazette, Vol. 78, No. 1, pp. 52-54, which document is herein incorporated by reference in its entirety, set a 2 mg/l limit on boron for discharge to certain inland waters. The U.S. effluent standard is also 2.0 mg/l, per the 1978 Effluent Standards of the National Pollution Control Commission, herein incorporated by reference in its entirety.
Where no local regulations exist for boron in irrigation water, it would stand to reason that any water supplied or sold for irrigation obviously should not contain boron in a concentration greater than the toxicity level for the plant under cultivation. For example, 2 mg/l boron in water is harmful for rice growth. Hence, to be on the safe side, the boron concentration for waters supplied to rice fields should be no greater than 2 mg/l.
In light of the foregoing, it can be seen that condensate produced from boron-containing geothermal steam poses a difficulty for geothermal plant operations. The boron originally present in the geothermal steam and/or in the steam flashed from the brine, ultimately, after passage through the turbine, becomes a component of the liquid steam condensate. The boron concentration in the steam condensate is oftentimes far in excess of the 2 mg/l limit, requiring as a practical consequence that neither water directly taken from the condenser nor from cooling tower blowdown be used for high value agricultural purposes or discharged into rivers, streams, and the like. Both the condensate and the cooling tower blowdown must eventually be re-injected into the earth for environmental protection.
Another problem exacerbated by the presence of boron is cooling tower "drift"--i.e., the moisture carried from the cooling tower. If the moisture emitted from the cooling tower contains boron, say in a concentration of 2 mg/l or more, its ultimate deposition upon the ground can cause plant distress or death in the immediate vicinity--depending on the sensitivity of the local plants to boron. This "drift" problem is especially acute with respect to geothermal power plants processing superheated steam taken directly from the geothermal formation. As such formations become depleted, the boron concentration in the steam produced increases, resulting in increased boron concentration in the drift.
Besides boron, another contaminant in geothermal steam which can accumulate in the condensate in undesirable concentrations for irrigation purposes or for discharge to inland waters or to the environment in general (e.g., by distribution upon the soil) is arsenic. Arsenic generally does not present as pervasive a problem for geothermal operations as boron since its concentration in geothermal steam is usually low, as is its concentration in the resulting steam condensate. Nevertheless, there are instances where the geothermal steam can contain arsenic in unusually large concentrations to produce a condensate containing arsenic in a concentration too high for discharge per local regulations. Generally, a limit of 0.1 mg/l will pertain for discharge to the environment, and 0.05 mg/l is the usual maximum for drinking water, although Pontius reports that values in the range of 0.002 to 0.020 mg/l for drinking water were under consideration in the U.S. in 1995. Plant toxicity to arsenic varies widely, van der Leeden et al. indicating that the tolerance varies from as much as 12 mg/l for Sudan grass to less than 0.05 mg/l for rice. Perhaps because rice cultivation is an important agricultural activity in the Philippines, the maximum value permitted for irrigation is 0.01 mg/l, per the Philippines NPCC Rules and Regulations of 1978 set forth hereinbefore. These same regulations, however, set a maximum limit of 0.05 mg/l for most other fresh surface waters, including water used as the source of public water supply.
Yet another contaminant of concern in geothermal steam is silicon. Although its ultimate presence in the condensate is benign, the more difficult problem occurs upstream of the condenser. In particular, silicon comes out of the steam as it passes through the turbine, depositing on the blades and thus reducing the turbine operating efficiency.